CS5165 datasheet(14/19 Pages) CHERRY

14 / 19 page

“Droop” Resistor for Adaptive Voltage Positioning

Adaptive voltage positioning is used to help keep the out-

put voltage within specification during load transients. To

implement adaptive voltage positioning a “Droop

Resistor” must be connected between the output inductor

and output capacitors and load. This resistor carries the full

load current and should be chosen so that both DC and AC

tolerance limits are met. An embedded PC trace resistor

has the distinct advantage of near zero cost implementa-

tion. However, this droop resistor can vary due to three

reasons: 1) the sheet resistivity variation causes the thick-

ness of the PCB layer to vary. 2) the mismatch of L/W, and

3) temperature variation.

1) Sheet Resistivity

for one ounce copper, the thickness variation is

typically 1.15 mil to 1.35 mil. Therefore the error due to

sheet resistivity is:

= 16%

2) Mismatch due to L/W

The variation in L/W is governed by variations due to

the PCB manufacturing process that affect the

geometry and the power dissipation capability of the

droop resistor. The error due to L/W mismatch is

typically 1%

3) Thermal Considerations

Due to I2 × R power losses the surface temperature of

the droop resistor will increase causing the resistance

to increase. Also, the ambient temperature variation

will contribute to the increase of the resistance,

according to the formula:

R = R20 [1+ α20(Τ−20)]

where:

R20 = resistance at 20˚C

α =

T= operating temperature

R = desired droop resistor value

For temperature T = 50˚C,

the % R change = 12%

Droop Resistor Tolerance

Tolerance due to sheet resistivity variation

16%

Tolerance due to L/W error

Tolerance due to temperature variation

12%

Total tolerance for droop resistor

29%

In order to determine the droop resistor value the nominal

voltage drop across it at full load has to be calculated. This

voltage drop has to be such that the output voltage full

load is above the minimum DC tolerance spec.

VDROOP(TYP) =

Example: for a 300MHz Pentium®II, the DC accuracy spec

is 2.74 < VCC(CORE) < 2.9V, and the AC accuracy spec is

2.67V < VCC(CORE) <2.93V. The CS5165 DAC output voltage

is +2.812V < VDAC < +2.868V. In order not to exceed the DC

accuracy spec, the voltage drop developed across the resis-

tor must be calculated as follows:

VDROOP(TYP)=

= 56mV

With the CS5165 DAC accuracy being 1%, the internal error

amplifier’s reference voltage is trimmed so that the output

voltage will be 40mV high at no load. With no load, there is

no DC drop across the resistor, producing an output volt-

age tracking the error amplifier output voltage, including

the offset. When the full load current is delivered, a drop of

-56mV is developed across the resistor. Therefore, the regu-

lator output is pre-positioned at 40mV above the nominal

output voltage before a load turn-on. The total voltage

drop due to a load step is ∆V-40mV and the deviation from

the nominal output voltage is 40mV smaller than it would

be if there was no droop resistor. Similarly at full load the

regulator output is pre-positioned at 16mV below the nom-

inal voltage before a load turn-off. the total voltage

increase due to a load turn-off is ∆V-16mV and the devia-

tion from the nominal output voltage is 16mV smaller than

it would be if there was no droop resistor. This is because

the output capacitors are pre-charged to value that is either

40mV above the nominal output voltage before a load turn-

on or, 16mV below the nominal output voltage before a

load turn-off (see figure 7).

Obviously, the larger the voltage drop across the droop

resistor ( the larger the resistance), the worse the DC and

load regulation, but the better the AC transient response.

Design Rules for Using a Droop Resistor

The basic equation for laying an embedded resistor is:

RAR = ρ ×

or R = ρ ×

(W × t)

2.812V-2.74V

1.3

[VDAC(MIN)-VDC PENTIUM®II(MIN)]

1+RDROOP(TOLERANCE)

[VDAC(MIN)-VDC(MIN)]

1+RDROOP(TOLERANCE)

0.00393

˚C

1.35 - 1.15

1.25

Application Information: continued

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